This invention relates generally to optical fibers, and more specifically to fluoride glass preforms from which long-length fibers can be drawn.
Optical waveguides have been known and used for some time. Optical waveguides may be employed, for example, in communications. There are three basic types of optical waveguides used in communications. For communications over relatively short distances, multimode, stepped index profile waveguides are used. These waveguides are generally used in conjunction with an incoherent light source such as an LED. For very long distance, high capacity communication systems, a single mode type waveguide is used. The single mode type waveguide supports propagation of only one mode due to a small diameter of the core. A solid state laser is usually the light source used with these fibers since it is one of the light sources capable of launching sufficient power into the single propagating mode. For intermediate distance applications, multimode fibers with a graded index profile are used. Pulse broadening, resulting from variation in path length of the various modes of light propagating down the waveguide, is reduced with a graded index profile as compared to the stepped index multimode fiber. Because of increasing interest in the infrared, fluoride glass fibers are of especial interest because of their exceptional response to infrared frequencies. The term fluoride glass is one known to those skilled in this art as any of a number of vitreous compounds of fluorine with the so called ZBLANs, e.g. zirconium, barium, lanthanum, aluminum, sodium, lead, gallium, lithium, etc.
Optical fibers are drawn from a core rod preform. To achieve drawn fibers without optical defects, it is necessary that the core rod preform from which the fibers are drawn is free from defects. Various methods have been proposed for making core rod preforms.
U.S. Pat. No. 4,519,826 to Tran discloses a method of making optical fibers having a fluoride glass cladding. Cladding glass is poured into a thermally-conductive, vertically disposed rotating mold. The mold is rotated about its vertical axis to allow the cladding class to coat the bore surface of the mold. The mold is then rapidly changed to a horizontal position while continuing rotation. The centrifugal force from rotation causes the mold to uniformly coat the bore surface of the mold. Rotation is continued until the temperature of the fluoride cladding glass approaches about the temperature of the mold, thus forming a cladding tube. Core glass melt may then be introduced into the cladding tube, forming a preform. The preform may then be drawn into an optical fiber.
In the traditional methods, difficulties are encountered in making a large-sized preform in which a long fiber may be drawn. The preform obtained in the past is usually about 10 mm in diameter and 100 mm or so in length. In practice, the actual length of the preform that can be used is even smaller. The length of a fiber which can be drawn out of the conventionally-made preforms is about 500 m at most. The primary reason for the difficulty in the manufacture of a large-sized preform rod by the conventional methods is that gas bubbles become trapped in the core glass melt when the latter is poured into the clad glass tube. In other words, when the core glass melt is poured into the clad glass tube, gas will remain as bubbles in the glass because of no escape for the gas in the cladding glass tube. As the cladding glass tube becomes longer and the core diameter becomes smaller, the core glass will further block gas escape, so gas bubbles will be more likely to remain in the preform rod.
A further problem with the conventional methods involves tensile strength of the fiber drawn out of the preform produced. The strength of a fiber is reduced by the scratches on the fiber surface. These scratches originate from those already present in the surface of the preform. With conventional manufacturing methods such as Tran's, the inner diameter surface of the cladding tube inevitably becomes uneven, requiring polishing before polishing of the core glass. The polishing leaves a number of scratches about the tube's inner diameter, with the result that the strength of the fiber drawn out of such a preform will also diminish.
A further problem associated with the conventional methods which initially form a clad glass tube is that heat is difficult to remove from the core glass melt as it is poured into the clad glass tube. Heat transfer is hindered by the clad glass tube. Heat removal is necessary to form a large fluoride glass preform. Heat transfer must be sufficiently quick to prevent crystallization of fluoride glass when the molten form is poured into the mold and solidifies therein. The quicker one can controllably remove heat without such crystallization, the larger one can make the preforms, and hence the more fiber one can draw.